The present invention is directed to an apparatus for initializing a brake control system. The apparatus of this invention is particularly useful in brake control systems such as antiskid systems.
One important class of modern brake control systems modulates braking action to achieve the desired level of braking. These systems include a modulator which generates a brake control signal as a time integral function of the difference between a reference value and a measured wheel characteristic, such as velocity or acceleration. The brake control signal is indicative of the desired modulated braking action.
For example, one type of such brake control system is an antiskid system which compares wheel velocity with a reference velocity to generate an error signal which is integrated over time. The integrated error signal is then used to generate a modulated brake control signal. This control signal is summed with other brake control signals and then used to control an antiskid valve which acts to reduce braking action on command. Other modulating brake control systems integrate the difference between measured wheel acceleration and a reference acceleration in order to generate the modulated brake control signal.
Such modulator based brake control systems provide significant advantages, primarily in terms of smooth, efficient braking and skid control. Because of these advantages, such systems have found widespread application or modern commerical jet aircraft. U.S. Pat. No. 3,724,916, issued Apr. 3, 1973 to Edgar A. Hirzel, discloses an early example of such a modulating brake control system.
However, modulator based systems have in the past suffered from certain disadvantages related to efficient modulator initialization. Because the modulated brake control signal is a time integral function of the braked wheel's rotational behavior, it does not ordinarily change level abruptly. This can result in excessive braking action and skidding during the initial brake application. For example, many prior art braking control systems routinely use an initial skid at aircraft touchdown to generate a large error signal to initialize the modulator and the brake control system.
This approach in effect induces an initial skid, the depth and duration of which serves to initialize the modulator. Typically, between one and two skids are needed to raise the output of the modulator to the point where it begins to operate properly. During this initialization period, wheel skid generally passes beyond the desired level of slip of establish an error signal large enough to initialize the modulator. This in turn results in reduced braking effectiveness, increased tire wear, and undesirable tire heating during the initialization period. In addition to reduced braking effectiveness during the initialization period, elevated tire temperature caused by the initialization skids can reduce the maximum braking force available after skid recovery.